Vehicular Adhoc Networks (VANET) is a type of road network that provides road safety and other infotainment applications to drivers and passengers for an effective and uninterrupted communication. In this network, the communication between the vehicles are equipped with the Road Side Units (RSU). VANET is the major component of Intelligent Transportation System (ITS). Research on Vehicular Adhoc network security presents many challenging issues to the researchers. The security mechanism available for VANETs are not highly effective. Hence it is as on time requirement of new and sophisticated security solutions. This paper mainly focuses on Intrusion Detection System (IDS) for VANETs to prevent Session Hijacking Attack (SHA). The work discusses on the total number of packet generated, sent, received and dropped with varying number of nodes with the help of Network Simulator-2 (NS2) and the inferred results are discussed.